1. Field of the Invention
The present invention relates to a micro-displacement element comprising a unimorph cantilever, as well as a scanning tunneling microscope (STM) and a large-capacity and high-density information processor using the micro-displacement element.
2. Related Background Art
In recent years, there has been a practice tendency that while the data recording capacity in an information processor is being increased year by year, the size of a recording unit is miniaturized and thus the recording density is heightened. For example, in a digital audio disk in which an optical recording system is utilized, the size of a recording unit is as small as 1 .mu.m.sup.2. In the background there is the active development of memory materials, and consequently, inexpensive and high-density recording media using organic thin films of organic dyes, photopolymers and the like have come out.
On the other hand, the STM has recently been developed by which the electronic structure of surface atoms of a conductor can be directly observed [G. Binning et al., Helvetica Physica Acta, 55, 726 (1982)], and as a result, the real space image can be measured with a high resolution irrespective of a single crystal or an amorphous substance. In addition, observation can be advantageously made at a low electric power without damaging a medium with current. Moreover, the STM can be operated even in the atmosphere and can be applied to various materials, and for these reasons, it is expected to be employed in many fields.
The STM utilizes the phenomenon that when a voltage is applied to between a metallic probe (a probe electrode) and a conductive material and the probe is then brought close to a position about 1 nanometer distant from the conductive material, a tunnel current flows therebetween. This current is very sensitive to a distance change between them, and therefore the surface information of the real space can be obtained by scanning a probe so as to constantly maintain the current or the average distance between them. In this case, the resolution in the surface direction is about 1 .ANG..
If the principle of this STM is utilized, a high-density recording, and reproduction can be sufficiently achieved in an atomic order (several angstroms). This as recording reproduction methods, there have been suggested, for example, a method which comprises changing the surface state of a suitable recording layer by the use of a particle beam (an electron beam or ion beam), or an energy beam including a high-energy electromagnetic wave such as X-ray and a visible or ultraviolet light, for recording information, and then reproducing it by the STM; and a method in which a material exhibiting a property of switching voltage-current characteristic with a memory effect, for example, a thin film of a .mu. electron type organic compound or a chalcogen compound is used as the recording layer, and recording and reproduction are carried out by using the STM (Japanese Patent Appln. Laid-Open No. 63-161552 and the like).
In the information processor utilizing the STM, it is important to control the distance between a probe and a recording medium in an angstrom order. In addition, it has been suggested to simultaneously drive many probes (multiplication of the probe) from the viewpoint of the function improvement of a recording/reproduction system, particularly a high-speed processing.
For this control, a lamination type piezoelectric element, a cylindrical piezoelectric element and the like attached on the probe or the medium have been heretofore used (U.S. Pat. No. 4668865). However, these elements are not suitable for integration, though the use of them permits obtaining a large displacement. Thus, it is inconvenient to use these elements in the multi-probe type information processor. In view of this, a method has been contrived in which a probe is attached on a cantilever having a length of about several hundreds .mu.m, and this cantilever is then driven by a piezoelectric element.
FIGS. 9 and 10 show an example in which a cantilever comprising a piezoelectric bimorph is formed on an Si substrate [Thomas R. Albrecht et al., J. Vac. Sci. Technol., A8, p. 317, 1990]. FIG. 9 is its perspective view, and as shown in this drawing, a cantilever is formed on an Si substrate 1 by laminating two-divided lower electrodes 3, a ZnO piezoelectric thin film 904, a medial electrode 902, a ZnO piezoelectric thin film 904 and two-divided upper electrodes, and a portion of the Si substrate under the cantilever is then removed by anisotropic etching so that the cantilever is held to be overhung from the edge of the Si substrate.
The cantilever comprising this piezoelectric bimorph is provided on the tip thereof with a metallic probe 6, which detects a tunnel current through an outgoing electrode. In this case, when voltages are independently applied to the four regions including two piezoelectric regions sandwiched between the upper electrodes 5 and the medial electrode 902 of the cantilever as well as two piezoelectric regions sandwiched between the lower electrodes 3 and the medial electrode 902 as shown in the sectional view of FIG. 10, the cantilever having the probe 6 can be independently moved.
However, when the cantilever having such a piezoelectric bimorph structure as seen in the conventional example is subjected to high-speed scanning, inconvenient mechanical vibration called host vibration is liable to occur on the cantilever, which makes a correct image observation difficult.
Additionally, in the case where a plurality of cantilevers are integrally arranged, there is a problem that some of them bend owing to the internal stress of the piezoelectric thin films or the electrode thin films. This is considered to be attributable to the non-uniformity of the piezoelectric thin films and the electrode thin films at the time of the formation of these films by a sputtering process or a vapor deposition process.
For the writing or reading of information in the information processor, it is necessary that all of the respective cantilevers operate normally. When the precision of these cantilevers is poor, an external compensating operation and the like are required to securing the precision.
In addition, since the cantilever takes the piezoelectric bimorph structure using the two piezoelectric layers, many manufacturing steps are necessary, which leads to the complication of the cantilever formation. As a result, it is difficult to control the stress of the thin films in the respective layers.